I thought I would pass this along, since there have been a number of posts on this forum from users having trouble getting GPO/JABB working with Logic 8 and GB 08. I was having similar problems, although GB 08 seemed to work with the Kontact 2 player. However, I couldn't get it to work with Logic Express 8.
So, I tried the suggestion on the sticky post of reinstalling the Kontact 2 Player. This actually made things worse, since after doing so, GB 08 no longer recognized KP2 (when attempting to select on of my defined GPO/JABB instruments, I got an error message saying something like 'KontactPla is unavailable...'). So, after poking around for a couple of hours, I noticed there was an update for KP2 (2.2.4) on the NI web site. I downloaded and installed this update and GB 08 began working again with K2. Furthermore, LE8 showed KontactPlayer2 when I ran its audio unit manager.
Unfortunately, this represents only a partial success, since no instruments from GPO or JABB show up directly in the LE8 instrument library. However, the instruments I have loaded into GB do show up (under "My GarageBand Settings"), so I am able to access all the instruments I have defined in GB when using LE8. There is also an instrument set called "Shared GarageBand Settings" that shows both GPO and JABB. However, under the GPO and JABB "folders" no instruments appear.
So, there seems to be a workaround for getting GPO and JABB instruments into LE8 (I assume this also applies to Logic Pro 8) by first defining them for GB08.
By the way, I am running Mac OS 10.4.11 on a dual processor G4 MDD. I have no way of knowing whether my experience applies to Leopard or to Intel Macs.